Partitioning of storage channels using programmable switches

ABSTRACT

A data storage system having a first storage channel, a first controller coupled to the first storage channel, a first storage device coupled to the first storage channel, a second storage channel, a second storage device coupled to the second storage channel, and a switch coupled to the first storage channel and the second storage channel. The switch separates the first storage channel from the second storage channel in a first state and connects the first storage channel and the second storage channel in a second state. Also described is a method of controlling a data storage system having a first storage channel, a first storage device coupled to the first storage channel, an operational controller coupled to the first storage channel, a second storage channel, a second storage device coupled to the second storage channel, and a switch coupled to the first storage channel and the second storage channel. The method includes detecting whether an operational controller is coupled to the second storage channel and if an operational controller is coupled to the second storage channel, then opening the switch.

This application is a continuation of U.S. patent application Ser. No. 09/162,109,” filed Sep. 28, 1998, which claims the benefit of U.S. provisional patent application Ser. No. 60/065,914, filed Nov. 14, 1997.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to data storage systems, specifically to a new storage channel architecture.

2. Related Art

Storage arrays often include a number of devices, such as disk drives, RAM disks, tape drives, and memory chips, connected to a controller by storage channels, such as a bus or cable. The controller provides an external access interface, managing transfer of data between the external access interface and the storage devices.

For disk drives and RAM disks, a storage array is commonly called a disk array, in which a disk controller connects a host computer to multiple disk drives. The disk controller may provide access to the actual drives in a JBOD (just a bunch of drives) configuration, or the disk controller may perform striping of data across the drives in a redundant array of independent disks (RAID) configuration. Storage channels often include AT Attachment (ATA), small computer system interface (SCSI), fibre channel, or storage system architecture (SSA). The external access interfaces often include industry standard architecture (ISA), bus or peripheral component interconnect (PCI) bus (for host adapters), SCSI, fibre channel, or SSA.

For tape drives, the storage array commonly includes individual tapes or tape silos. The controller may provide data striping capability across the tapes. The storage channels and external access interfaces are usually the same as for disk drives.

For memory chip storage devices, the storage array commonly is the main processor memory, cache memory, or other memory subsystem. The controller commonly performs error detection and correction (parity and ECC) and provides data striping (usually called interleaving). The storage channels are the memory buses. The external access interfaces are commonly PCI bus or processor bus.

In order to maintain access to the storage devices in the event of a single controller failure (to provide high-availability), two controllers may be attached to the same storage devices, in a ‘dual-controller’ configuration. One controller may provide access to one set of storage devices and the other controller may provide access to another set of storage devices. Such a configuration is to provide access to the storage devices from the surviving controller should one controller fail.

FIG. 1 (Prior Art) shows a storage array with two controllers 10, each with one external access interface 12. Both controllers 10 are connected via three shared storage channels, e.g., channel 14, to six (single-ported SCSI drive) storage devices, e.g., storage devices 16. FIG. 2 (Prior Art) shows a storage array with two controllers 10, each with one external access interface 12. Both controllers 10 are connected via two shared storage channels, e.g., storage channel 14, to six (dual-ported FC-AL drive) storage devices, e.g., storage device 16. In both architectures all of the storage channels are connected to both controllers to allow either controller to access any storage device via any storage channel. For dual-ported storage devices, this configuration allows a single storage channel to fail, while still retaining access to the drive from either controller.

A particular controller supports a maximum number of storage channels. This determines the maximum bandwidth, ‘B’, for data transfers between the storage devices and the controller. In a dual-controller configuration, the two controllers could potentially support double this bandwidth, ‘B×2’; however, since the storage channels are connected to both controllers for high availability, these shared storage channels only support a combined bandwidth of ‘B’.

Normally about half of the maximum bandwidth, ‘B/2’, is used by each controller, since each controller only accesses its own storage devices. Only in the case of a controller failure, does the surviving controller use all its potential bandwidth ‘B’ on all of the storage channels. So in a normal non-failure case, half of the storage channel bandwidth on a controller, and its associated hardware capability, is unused thereby increasing the cost of the controller.

Furthermore, when additional storage devices are added to a storage array, the maximum bandwidth does not change, so at some point additional storage devices merely add to the total storage data capacity, and not to the storage array performance.

New storage devices may be connected to a new pair of controllers. This however creates a second storage array independent of the original storage array. Having multiple storage arrays increases the complexity and cost of the overall storage subsystem, both in terms of administration and maintenance. Furthermore, a controller in one storage array cannot access a storage device in the other storage array. Therefore, with two independent storage arrays an external switching mechanism (e.g. Fibre Channel switch) may be needed on the external access interfaces to allow an external access interface to transfer data to and from any of the storage devices.

SUMMARY

According to an embodiment of the invention, a data storage system has a first storage channel, a first controller coupled to the first storage channel, a first storage device coupled to the first storage channel, a second storage channel, a second storage device coupled to the second storage channel, and a switch coupled to the first storage channel and the second storage channel. The switch separates the first storage channel from the second storage channel in a first state and connects the first storage channel and the second storage channel in a second state.

According to another embodiment of the invention, a data storage system, comprises a first storage channel, a first controller coupled to the first storage channel, a first storage device coupled to the first storage channel, a second storage channel, a second controller coupled to the second storage channel, a second storage device coupled to the second storage channel, a third storage channel coupled to the first controller and the first storage device, a fourth storage channel coupled to the second controller and the second storage device, and a switch coupled to the first storage channel and the second storage channel. The switch separates the first storage channel from the second storage channel in a first state and connects the first storage channel and the second storage channel in a second state.

According to yet another embodiment of the invention, a data storage system comprises a first storage channel, a first storage device coupled to the first storage channel, and a switch coupled to the first storage channel. The switch is coupled to an interface to couple to a second storage channel that is coupled to a second storage device. The switch separates the first storage channel from the second storage channel in a first state and connects the first storage channel and the second storage channel in a second state.

According to yet another embodiment of the invention, a data storage system, comprises a fibre channel loop, a first plurality of storage devices coupled to the fibre channel loop, a loop resiliency circuit coupled to the fibre channel loop. The loop resiliency circuit has an interface to couple to a second storage channel that is coupled to a second plurality of storage devices, and the loop resiliency circuit is to separate the fibre channel loop from the second storage channel in a first state and to connect the fibre channel loop and the second storage channel in a second state.

Yet another embodiment of the invention relates to a method of controlling a data storage system having a first storage channel, a first storage device coupled to the first storage channel, an operational controller coupled to the first storage channel, a second storage channel, a second storage device coupled to the second storage channel, and a switch coupled to the first storage channel and the second storage channel. The method includes detecting whether an operational controller is coupled to the second storage channel and if an operational controller is coupled to the second storage channel, then opening the switch.

According to yet another embodiment of the invention, a data storage system comprises a first storage channel, a first controller coupled to the first storage channel, a first storage device coupled to the first storage channel, a second storage channel, a second storage device coupled to the second storage channel, a switch coupled to the first storage channel and the second storage channel, and logic that controls the switch according to whether an operational controller is coupled to the second storage channel.

DESCRIPTION OF DRAWINGS

FIG. 1 (Prior Art) shows an architecture of a SCSI based RAID storage system.

FIG. 2 (Prior Art) shows an architecture of an FC-AL based RAID storage system.

FIG. 3 shows two storage arrays interconnected by a programmable switch.

FIG. 4 shows two storage arrays with four independent storage channels.

FIG. 5 shows two storage arrays with two shared storage channels.

FIG. 6 shows two storage arrays with two independent storage channels and one shared storage channel.

FIG. 7 shows four storage arrays, only two of which have controllers.

FIG. 8 shows four storage arrays with four independent storage channels and two shared storage channels.

FIG. 9 shows a flowchart of software code that automatically sets switch configuration.

FIG. 10 shows an embodiment of a disk array, with disk drives, a controller card, and loop cards.

FIG. 11 shows a detailed block diagram of a loop card, with LRC circuits and hardware register circuit.

FIG. 12 shows a detailed block diagram of an LRC circuit.

FIG. 13 shows a detailed block diagram of a hardware register circuit.

DETAILED DESCRIPTION

This invention allows a single storage channel to be divided into multiple independent partitions using programmable hardware switches. A storage array consisting of a set of storage devices and a controller may be connected to each partition. When a switch is closed, it combines the two adjacent partitions into one partition allowing data transfer to occur across the switch. When a switch is open, it splits a partition into two, thereby doubling the total storage channel bandwidth.

FIG. 3 shows two connected storage arrays in an architecture with two storage channels and dual-ported storage devices. Storage array 20 a has a controller 10 a with one external access interface 12 a. The controller 10 a is connected to its dual-ported storage devices, e.g., 16 a, via two storage channels 14 a 1 and 14 a 2. Similarly, storage array 20 b has a controller 10 b with one external access interface 12 b. The controller 10 b is connected to its dual-ported storage devices, e.g., 16 b, via two storage channels 14 b 1 and 14 b 2. Storage channel 14 a 1 is connected to storage channel 14 b 1 via a hardware switch 18 a 1. Similarly storage channel 14 a 2 is connected to storage channel 14 b 2 via a hardware switch 18 a 2. These two switches 18 a 1 and 18 a 2 connect the two storage arrays 20 a and 20 b. There are hardware switches 18 on the other ends of the four storage channels 14 a 1, 14 a 2, 14 b 1, and 14 b 2 which are not connected to other storage arrays.

This configuration helps each controller to use its potential bandwidth both when the controller is servicing only a portion of the drives and when it is servicing all the drives. This configuration thus helps eliminate the expense of unused hardware capability of the prior art. The total bandwidth of a storage array increases linearly with the number of controllers. According to embodiments of the invention, not only storage devices may be added to an existing array, but controllers may also be added to an existing array, thereby increasing performance both from existing storage devices and from new storage devices. When new controllers or storage devices are added to an existing storage array, according to one aspect of the invention, the result is still a single storage array.

An advantage of an embodiment of the invention is that the addition of controllers and storage devices may be achieved without physical movement or recabling, and without interruption in data access (no system down-time). According to an aspect of the invention, a storage array has more than two controllers. Since total bandwidth may increase linearly with the number of controllers, the total performance of the storage array can be scaled. Furthermore, according to an aspect of the invention, every controller has the capability of accessing every storage device, eliminating the need for an external switching mechanism.

FIG. 4 shows the two storage arrays connected in a normal configuration in which each controller is accessing its own storage devices. Since the switches 18 a 1 and 18 a 2 between the storage arrays are open, there are a total of four independent storage channels 14 a 1, 14 a 2, 14 b 1, and 14 b 2, each of which can operate at full bandwidth.

FIG. 5 shows two storage arrays where one controller 10 b has failed. The switches 18 a 1 and 18 a 2 between the storage arrays are closed so that there are only two storage channels 14 ab 1 and 14 ab 2. The surviving controller 10 a has access to all the storage devices using both storage channels 14 ab 1 and 14 ab 2.

The switches on the storage channels may be configured differently (opened or closed). FIG. 6 shows two storage arrays where switch 18 a 1 is closed to form one storage channel 14 ab 1, and switch 18 a 2 is opened to form two independent storage channels 14 a 2 and 14 b 2. This gives controller 10 a its own independent storage channel 14 a 2 for accessing its own storage devices. It also gives controller 10 b its own independent storage channel 14 b 2 for accessing its own storage devices. Both controllers 10 a and 10 b can also access each other's storage devices using the shared storage channel 14 ab 1. This shared storage channel 14 ab 1 may also be used by the controllers 10 a and 10 b to communicate with each other or to transfer data between each other. One example where data transfer between controllers is useful is for cache mirroring in redundant array of independent disks (RAID) controllers. For performance reasons, a RAID controller commonly caches data within the controller before writing it to the drives. In order to protect against controller failure, this data is mirrored (or copied) in the other controller. The shared storage channel could be dedicated for cache mirroring between the controllers.

In one embodiment of the invention, some storage arrays do not have a controller. FIG. 7 shows four storage arrays, two of which have controllers, and two of which only have storage devices. Storage array 20 a has a controller 10 a and storage array 20 d has a controller 10 d. Storage arrays 20 b and 20 c do not have controllers. The switches 18 a 1, 18 b 1, 18 c 1, 18 a 2, and 18 c 2 are closed and the switch 18 b 2 is open. Storage channel 14 ad 1 is shared between the two controllers 10 a and 10 d (e.g. for cache mirroring). Controller 10 a also has a storage channel 14 ab 2 for access to the storage devices in storage array 20 a and 20 b. Controller 10 d also has a storage channel 14 cd 2 for access to the storage devices in storage array 20 c and 20 d.

In order to add performance to such a storage system, controllers may be added to the storage arrays which do not have controllers. Controllers may be added and the switches are updated correspondingly to provide each controller with the required bandwidth. FIG. 8 shows an example system with two more controllers 10 b and 10 c added. Three switches 18 b 1, 18 a 2, and 18 c 2 have been configured so that each pair of controllers share a common storage channel, and each controller also has a storage channel to its own storage devices. Controllers 10 a and 10 b share a storage channel 14 ab 1. Controllers 10 c and 10 d share a storage channel 14 cd 1. Each controller 10 a, 10 b, 10 c, and 10 d also have storage channels 14 a 2, 14 b 2, 14 c 2, and 14 d 2 (respectively) for accessing their own drives.

Additional storage arrays may also be added to an existing set of storage arrays by attaching them to open switches which only have one storage channel partition attached. In FIG. 7 for example, another storage array could be attached to the right ‘end’ of the set of storage arrays, and the switches 18 d 1 and 18 d 2 updated appropriately.

The reconfiguration (opening/closing) of the switches may either be done manually by user intervention or automatically. For example, FIG. 9 shows a flowchart software code that automatically sets the switch configuration based on whether a controller is operational or failed, and whether cache mirroring between controllers is enabled. The determination whether a controller is operational may be based on various mechanisms, such as heartbeat messages between controllers. Whether cache mirroring is enabled may be a static configuration parameter.

First test whether the other controller is operational 100. If not, then both switches 18 a 1 and 18 a 2 are closed 108 so the controller can access all the storage devices, as shown in FIG. 5. Thereafter, a periodic polling checks whether the other controller becomes operational 112. If the other controller is operational, either from 100 or 112, then a check is made to see if cache mirroring is enabled 102. If cache mirroring is enabled 102, then switch 18 a 1 is closed and switch 18 a 2 is opened 104. The closure of switch 18 a 1 creates a shared storage channel 14 ab 1 between the controllers, and the opening of switch 18 a 2 creates two independent storage channels 14 a 2 and 14 b 2, as shown in FIG. 6. If cache mirroring is not enabled 102, then both switches 18 a 1 and 18 a 2 are opened 106. This creates four independent storage channels 14 a 1, 14 a 2, 14 b 1, and 14 b 2, as shown in FIG. 4. Thereafter, a periodic polling checks if the other controller becomes non-operational 110, and if so, the switches are closed 108 to allow the surviving controller access to all the storage devices.

An embodiment of the software checks for the presence of new controllers, in addition to just checking whether existing controllers are operational or failed. Then, if new controllers are added to an existing system, the hardware switches are automatically reconfigured. For example, according to an embodiment of the invention, adding two new controllers to the system in FIG. 7 automatically results in a configuration as in FIG. 8.

An advantage of an embodiment of the invention is the ability to create high availability configurations. For example, according to an embodiment of the invention, a single storage device has more than two controllers, e.g., three controllers, four controllers, or more, according to the needs of the system.

The following example embodiment is an application with disk arrays. Fibre Channel Arbitrated Loop (FC-AL) is used for the storage channels.

FIG. 10 shows a storage array which is a single unit 32 which supports nine disk drives, e.g., disk drive 16, one controller card 10, and two loop cards 30-1 and 30-2. The unit 32 also includes power/packaging/cooling (not shown). The drives, e.g., drive 16, controller card 10, and loop cards 30-1 and 30-2 may be hot-plugged into and hot-swapped from a passive backplane (not shown) providing high-availability.

The controller card 10 and every disk drive, e.g., disk drive 16, is dual-ported and connected to two independent FC-AL loops 14-1 and 14-2. The hardware circuitry for loop 141 is on loop card 30-1. The hardware circuitry for loop 14-2 is on loop card 30-2.

The controller card 10 has an external access interface 12. The example implementation supports an FC-AL access interface for connection to host computers. The controller card 10 supports standard functions of a RAID controller.

Loop card 30-1 has two external connectors 28L1 and 28R1, which support cables (not shown) for connecting to other units. Similarly loop card 30-2 has two external connectors 28L2 and 28R2. A unit may be cabled to two adjacent units (on left and right) via the two connectors on each loop card, such that 28L1 on one unit connects to 28R1 on the unit on the left, and 28L2 on one unit connects to 28R2 on the unit on the left. Each external connector 28L1, 28R1, 28L2, and 28R2 and each interconnection cable supports an FC-AL loop and a serial communication channel.

Status signals 26 from drives, e.g., drive 16, from the controller card 10, and from the loop cards 30-1 and 30-2 indicate whether those components are physically present. These status signals 26 are routed to both loop cards 30-1 and 30-2. A separate serial communication channel 24C runs between the controller 10 and each loop card 30-1 and 30-2.

Loop Card Detail

FIG. 11 shows a block diagram of a loop card 30. Loop resiliency circuits (LRC, also known as port bypass circuits) 38 are used to connect the controller 10 and the disk drives, e.g., disk drive 16, to the fibre channel loop 14. An additional two LRCs 38L and 38R are used on each loop to connect adjacent units to either side (on left and right) of this unit, via the loop card connectors 28L and 28R. The LRCs 38L and 38R implement the programmable hardware switches described in this invention.

FIG. 12 shows a block diagram of a typical LRC 38 and illustrates that this device can be switched by a signal 50. If the signal 50 is asserted, the device (not shown) attached via signals 42 and 44 is bypassed and the input serial bit stream 40 is routed directly to the output 46. If the signal 50 is asserted, the device (not shown) attached via signals 42 and 44 is attached to the loop by routing the input bit stream 40 to the device via signal 42, and routing the returning bit stream 44 from the device to the output 46.

As shown in FIG. 11, the LRC control signals 50 are driven, via a control/sense bus 34, by a hardware register circuit 36. The ‘component present’ status signals 26 from the drives, controller cards, and loop cards are also routed, via a control/sense bus 34, to the hardware register circuit 36. There is also a status signal 26 from each of the left and right connectors, 28L and 28R, indicating whether a cable is present. These status signals 26 along with various control and status signals from the power/cooling system (not shown) are also routed, via the control/sense bus 34, to the hardware register circuit 36.

Three bi-directional serial communication channels, 24C, 24L, and 24R, are connected to the hardware register circuit 36. Channel 24C runs to the controller 10. Channel 24L is connected to the loop card connector 28L for connection to an adjacent left unit. Channel 24R is connected to another loop card connector 28R for connection to an adjacent right unit.

Hardware Register Circuit Detail

FIG. 13 shows a block diagram of the Hardware Register Circuit 36 in loop card 30. The hardware registers 56 are connected to the status and control signals 34. The UART in an 8051 microcontroller 52 is connected to the controller communication channel 24C. A separate dual UART 58 provides serial communication channels 24L and 24R for communication with the adjacent units. The FLASH ROM 54 contains 8051 firmware. A data bus 60 connects the hardware registers 56, dual UART 58, FLASH ROM 54, and microcontroller 52.

The firmware in the 8051 microcontroller 52 implements a serial protocol on the serial communication channels 24R, 24L, and 24C. This protocol allows for the reading and writing of the hardware registers 56 from any serial channel 24R, 24L, or 24C. The FLASH ROM 54 may also be reprogrammed via the serial protocol.

Operation

This description refers to FIG. 10 unless otherwise noted. Software is used to automatically reconfigure the loops 14-1 and 14-2 based on the presence of units 32, drives 16, and controllers 10. In this example implementation, the software is executed on the controller cards 10 in the units 32. This allows for easy modification and greater flexibility. Alternatively, processes executed on the controller cards could also be executed by the 8051 microcontroller 52 (FIG. 13) firmware on the loop cards 30-1 and 30-2.

A software task periodically polls the status signals 26 to determine what components are present in unit 32. This polling is done via a serial protocol which supports the reading and writing of the hardware registers 56 (FIG. 13) in a unit. The controller 10 communicates with the 8051 microcontrollers 52 (FIG. 13) on the local unit loop cards 30-1 and 30-2 via the serial communication channels 24C. If the request is for another unit, these 8051 microcontrollers forward the request to the next unit via the serial communication channels 24L and 24R (FIG. 13) on the unit interconnection cables. If necessary, those 8051 controllers in turn forward the request to the next unit.

The LRC circuits 38L and 38R (FIG. 11) are updated accordingly to various rules depending on the configuration of the units and whether cache mirroring is required between controllers. For example, when a new unit is added which does not have a controller card, the LRC circuits 38L and 38R are updated to connect that unit's loops to the existing loops on the unit to which it was attached. This is done by closing the switch, either 38L or 38R (FIG. 11) depending on the position of the new unit. Alternatively if a controller card 10 is added to an existing unit 32, either manually under user control or automatically, the LRC circuits 38L and 38R may be updated to provide that new controller an independent drive loop for its own drives. This is done by switching the LRC circuits 38L and 38R (FIG. 11) accordingly.

Alternative Embodiments

While the above description details particular implementations, for example, with respect to Fibre Channel disk storage arrays, this should not be construed as a limitation on the scope of the invention. Many other variations are possible, some examples of which follow.

Embodiments of the invention include configurations with any number of storage devices and any type of storage device, such as RAM disks, tape drives, and memory devices. Embodiments of the invention include single or multiple storage channels, single or multiple-ported storage devices, and varying topologies.

Alternative topologies include 2-dimensional, 3-dimensional, or N-dimensional arrays with various interconnection architectures, such as N—N and hypercube in addition to single-dimensional array of interconnected storage arrays.

Another topology could be a closed ring of storage arrays in which there are no free ‘ends’ with unattached switches. For 1-dimensional arrays, a ring forms a circle. For N-dimensional arrays, a ring may form a donut-shape or toroid.

Another topology is a star configuration of storage arrays, again in any number of dimensions. Other topologies will become apparent from consideration of the drawings.

Embodiments of the invention include configurations with any type of storage channel, such as ATA, SCSI, and SSA in addition to FC-AL storage channels.

The hardware switches in this invention may be of various forms. In order to allow automatic configuration of these switches, in some embodiments the switches comprise electronic devices. It is also possible that these switches be mechanical devices that require user intervention to configure. In addition to the switch, the storage arrays may be connected via various types of interconnection, such as a bus, connector, or cable.

The controllers and storage devices need not be removable or replaceable in order to benefit from this invention. The hardware switches allow for alternative access paths to the storage devices from any access interface. The hardware switches also allow for partitioning of the storage channels in various configurations for scaling of bandwidth to suit some data bandwidth requirement.

Various embodiments have been disclosed herein for the purpose of illustration. Modifications and substitutions are possible without departing from the spirit of the invention. Accordingly, the scope of the invention should not be restricted to the embodiments illustrated, but should be determined by the appended claims and their legal equivalents. 

1. A data storage system, comprising: a first storage channel for providing a connection to a first storage device; a first controller coupled to the first storage channel; a second storage channel for providing a connection to a second storage device; a switch coupled to separate the first storage channel from the second storage channel in a first state and to connect the first storage channel to the second storage channel in a second state; and a control circuit configured to cause entry of the switch in the second state in response to detecting a failure of the first controller.
 2. The data storage system as recited in claim 1, further comprising the first storage device connected to the first storage channel, and the second storage device connected to the second storage channel.
 3. The data storage system as recited in claim 1 further comprising a second controller coupled to the second storage channel, wherein said control circuit is further configured to cause entry of the switch in the first state in response to detecting a connection of a second controller to the second storage channel.
 4. The data storage system as recited in claim 1 wherein the control circuit is configured to periodically poll the first controller to determine whether the first controller is operational.
 5. The data storage system as recited in claim 1 wherein the control circuit includes a microcontroller.
 6. The data storage system as recited in claim 1 wherein the switch is a programmable hardware switch.
 7. The data storage system as recited in claim 1 wherein said first controller is configured to implement RAID functionality.
 8. The data storage system as recited in claim 2, wherein the first storage device comprises a disk drive.
 9. The data storage system as recited in claim 2 further comprising a second controller coupled to the second storage channel, wherein said control circuit is further configured to cause entry of the switch in the first state in response to detecting a connection of a second controller to the second storage channel.
 10. The data storage system as recited in claim 2 further comprising: a third storage channel coupled to the first controller and the first storage device; and a fourth storage channel coupled to the second storage device.
 11. The data storage system as recited in claim 2 further comprising a second controller coupled to the second storage channel.
 12. The data storage system as recited in claim 8, wherein the first storage channel comprises a Fibre Channel bus.
 13. The data storage system as recited in claim 10 further comprising: a second switch coupled to the third storage channel and the fourth storage channel, the second switch separating the third storage channel from the fourth storage channel in a first state and connecting the third storage channel and the fourth storage channel in a second state.
 14. The data storage system as recited in claim 10 further comprising: a second controller coupled to the second storage channel and to the fourth storage channel; wherein the second controller includes memory for mirroring data in the first controller.
 15. The data storage system as recited in claim 11 further comprising: a third storage channel; and a second switch coupled to separate the second storage channel from the third storage channel in a first state and to connect the second storage channel to the third storage channel in a second state.
 16. The data storage system as recited in claim 12, further comprising a loop resiliency circuit connecting said first controller to said first storage channel.
 17. The data storage system as recited in claim 14 wherein the control circuit is further configured to cause entry of the switch in the second state in response to an indication of a mirroring mode.
 18. The data storage system as recited in claim 15 further comprising: a third controller coupled to the third storage channel; and a third storage device coupled to the third storage channel.
 19. A data storage system, comprising: a first storage channel for providing a connection to a first storage device; a first controller coupled to the first storage channel; a second storage channel for providing a connection to a second storage device; a second controller coupled to the second storage channel; and a switch coupled to separate the first storage channel from the second storage channel in a first state and to connect the first storage channel to the second storage channel in a second state; wherein each of the first and second controllers is configured to cause entry of the switch in the second state in response to detecting a failure of the other controller.
 20. The data storage system as recited in claim 19, further comprising the first storage device connected to the first storage channel, and the second storage device connected to the second storage channel.
 21. The data storage system as recited in claim 19 further comprising a control circuit configured to cause entry of the switch in the first state in response to detecting a connection of the second controller to the second storage channel.
 22. The data storage system as recited in claim 19 wherein the second controller includes a control circuit configured to periodically poll the first controller to determine whether the first controller is operational.
 23. The data storage system as recited in claim 19 wherein the switch is a programmable hardware switch.
 24. The data storage system as recited in claim 19 wherein said first controller is configured to implement RAID functionality.
 25. The data storage system as recited in claim 20, wherein the first storage device comprises a disk drive.
 26. The data storage system as recited in claim 20 further comprising: a third storage channel coupled to the first controller and the first storage device; and a fourth storage channel coupled to the second storage device.
 27. The data storage system as recited in claim 22 wherein the control circuit includes a microcontroller.
 28. The data storage system as recited in claim 25, wherein the first storage channel comprises a Fibre Channel bus.
 29. The data storage system as recited in claim 26 further comprising: a second switch coupled to the third storage channel and the fourth storage channel, the second switch separating the third storage channel from the fourth storage channel in a first state and connecting the third storage channel and the fourth storage channel in a second state.
 30. The data storage system as recited in claim 28, further comprising a loop resiliency circuit connecting said first controller to said first storage channel.
 31. The data storage system as recited in claim 29 wherein the second controller includes memory for mirroring data in the first controller.
 32. The data storage system as recited in claim 31 further comprising a control circuit configured to cause entry of the second switch in the second state in response to enabling a mirroring mode.
 33. A data storage system, comprising: a first Fibre Channel loop; a first controller coupled to the first Fibre Channel loop, wherein the first controller is operable according to RAID functionality; a first storage device coupled to the first Fibre Channel loop; a second Fibre Channel loop a second storage device coupled to the second Fibre Channel loop; a switch coupled to separate the first Fibre Channel loop from the second Fibre Channel loop in a first state and to connect the first Fibre Channel loop to the second Fibre Channel loop in a second state; and a circuit configured to cause the switch to enter the second state in response to detecting a failure of the first controller.
 34. The data storage system as recited in claim 33, wherein the first storage device comprises a disk drive.
 35. The data storage system as recited in claim 33, further comprising a second controller coupled to the second Fibre Channel loop, wherein said control circuit is further configured to cause entry of the switch in the first state in response to detecting a connection of the second controller to the second Fibre Channel loop.
 36. The data storage system as recited in claim 34, further comprising a loop resiliency circuit connecting said first controller to said first Fibre Channel loop.
 37. A method for controlling a data storage system including a first controller coupled to a first storage device through a first storage channel and a second controller coupled to a second storage device through a second storage channel, the method comprising: setting a switch in a first state to separate the first storage channel from the second storage channel; detecting a failure of the first controller; and setting the switch in a second state to connect the first storage channel to the second storage channel in response to detecting the failure of the first controller.
 38. The method as recited in claim 37, wherein the first storage device comprises a disk drive.
 39. The method as recited in claim 37 further comprising setting the switch in the first state in response to detecting a connection of the second controller to the second storage channel.
 40. The method as recited in claim 38, wherein the first storage channel comprises a Fibre Channel bus.
 41. A data storage system, comprising: a first storage channel means; a first controller means coupled to the first storage channel means; a first storage means coupled to the first storage channel means; a second storage channel means; a second controller means coupled to the second storage channel means; a second storage means coupled to the second storage channel means; means for separating the first storage channel means from the second storage channel means; means for detecting a failure of the first controller means; and means for connecting the first storage channel means to the second storage channel means in response to detecting the failure of the first controller means.
 42. The data storage system as recited in claim 41 further comprising means for separating the first storage channel means from the second storage channel means in response to detecting a connection of the second controller means to the second storage channel means.
 43. A data storage system, comprising: a first storage channel for providing a connection to a first storage device; a first controller coupled to the first storage channel; a second storage channel for providing a connection to a second storage device; a switch coupled to separate the first storage channel from the second storage channel in a first state and to connect the first storage channel to the second storage channel in a second state; and a control circuit configured to cause entry of the switch in the first state in response to detecting a connection of a second controller to the second storage channel.
 44. The data storage system as recited in claim 43, further comprising the first storage device connected to the first storage channel, and the second storage device connected to the second storage channel.
 45. The data storage system as recited in claim 43 wherein the control circuit includes a microcontroller.
 46. The data storage system as recited in claim 43 wherein the switch is a programmable hardware switch.
 47. The data storage system as recited in claim 43 wherein said first controller is configured to implement RAID functionality.
 48. The data storage system as recited in claim 44 further comprising: a third storage channel; and a second switch coupled to separate the second storage channel from the third storage channel in a first state and to connect the second storage channel to the third storage channel in a second state.
 49. The data storage system as recited in claim 44, wherein the first storage device comprises a disk drive.
 50. The data storage system as recited in claim 44, further comprising: a third storage channel coupled to the first controller and the first storage device; and a fourth storage channel coupled to the second storage device.
 51. The data storage system as recited in claim 48 further comprising: a second controller couple to the second storage channel; a third controller coupled to the third storage channel; and a third storage device coupled to the third storage channel.
 52. The data storage system as recited in claim 49, wherein the first storage channel comprises a Fibre Channel bus.
 53. The data storage system as recited in claim 50 further comprising: a second switch coupled to the third storage channel and the fourth storage channel, the second switch separating the third storage channel from the fourth storage channel in a first state and connecting the third storage channel and the fourth storage channel in a second state.
 54. The data storage system as recited in claim 52, further comprising a loop resiliency circuit connecting said first controller to said first storage channel.
 55. The data storage system as recited in claim 53 wherein the second controller is coupled to the second storage channel and includes memory for mirroring data in the first controller.
 56. The data storage system as recited in claim 55 wherein the control circuit is further configured to cause entry of the switch in the second state in response to an indication of a mirroring mode.
 57. A data storage system, comprising: a first Fibre Channel loop; a first controller coupled to the first Fibre Channel loop, wherein the first controller is operable according to RAID functionality; a first storage device coupled to the first Fibre Channel loop; a second Fibre Channel loop a second storage device coupled to the second Fibre Channel loop; a switch coupled to separate the first Fibre Channel loop from the second Fibre Channel loop in a first state and to connect the first Fibre Channel loop to the second Fibre Channel loop in a second state; and a circuit configured to cause the switch to enter the first state in response to detecting a connection of a second controller to the second Fibre Channel loop.
 58. The data storage system as recited in claim 57, wherein the first storage device comprises a disk drive.
 59. The data storage system as recited in claim 58, further comprising a loop resiliency circuit connecting said first controller to said first Fibre Channel loop.
 60. A system, comprising: a first storage channel for providing a connection to a first storage device; a first controller coupled to the first storage channel; a second storage channel for providing a connection to a second storage device; a second controller coupled to the second storage channel; and a switch coupled to separate the first storage channel from the second storage channel in a first state and to connect the first storage channel to the second storage channel in a second state; wherein each of the first and second controllers is configured to cause entry of the switch in the first state in response to detecting presence of the other controller.
 61. A method for controlling a data storage system including a first controller coupled to a first storage device through a first storage channel, the method comprising: setting a switch in a first state to connect the first storage channel to a second storage channel; detecting a presence of a second controller coupled to the second storage channel; and setting the switch in a second state to separate the first storage channel from the second storage channel in response to detecting the presence of the second controller.
 62. The method as recited in claim 61, wherein the first storage device comprises a disk drive.
 63. The method as recited in claim 61, wherein a second storage device is coupled to the second storage channel.
 64. The method as recited in claim 62, wherein the first storage channel comprises a Fibre Channel bus.
 65. A data storage system, comprising: a first storage channel means; a first controller means coupled to the first storage channel means; a first storage means coupled to the first storage channel means; a second storage channel means; a second controller means coupled to the second storage channel means; a second storage means coupled to the second storage channel means; means for connecting the first storage channel means to the second storage channel means; means for detecting a presence of the second controller means; and means for separating the first storage channel means from the second storage channel means in response to detecting the presence of the second controller means. 